苏北平原2.58Ma以来的海陆环境演变历史——宝应钻孔沉积物的常量元素记录

MAJOR ELEMENT RECORDS OF LAND-SEA INTERACTION AND EVOLUTION IN THE PAST 2.58Ma FROM THE BAOYING BOREHOLE SEDIMENTS,NORTHERN JIANGSU PLAIN,CHINA

苏北盆地是一个以箕状断陷为特点的复合型沉积盆地。文章根据2004年夏采集的苏北盆地宝应钻孔(BY1)97m沉积物的124个样品11种常量元素S iO2,A l2O3,TiO2,Fe2O3,MgO,MnO,CaO,K2O,Na2O,P2O5和SO2的地球化学特征分析,探讨了海相沉积环境与陆相沉积环境的元素分布特点、成因、相互之间的联系和物质来源。根据沉积物常量元素含量的垂向变化,结合沉积相、岩性特征、有孔虫分析、古地磁及14C定年,分析了苏北盆地2.58M a以来不同阶段海陆交互作用下的常量元素变化特点,反映了苏北平原在第四纪曾经历了4次海水淹覆,海-陆交互堆积作用明显,经滨浅海→泻湖→湖泊→洼地,最后由河流冲积物覆盖成陆。

The Northern Jiangsu Plain along the Yellow Sea is located on the northern side of the Changjiang River and south of the Cape Lan-Shan Tou, 32°10′～35°05′N, and 118°40′～120°30′E. Historical records and previous studies have indicated that the east part of the plain was formed from the sea during a Holocene high sea level period at ca. 5000aB. P. The present study is focused on the west part of the plain which is located landward of the shell beach ridges and to the west of the Grand Canal. The BY1 Borehole was drilled 145m deep at Wangzhigang Town of Baoying County, in the central lowland area of the plain and between the lakes and the outer coastal ridges. 97m of clay and silty core sediments were used to interpret the North Jiangsu Plain＇s geomorphologic history since 2.58Ma by a muhidisciplinary study of sedimentary facies, microfossils and geochemistry. Major elements of SiO2, Al2O3, TiO2, Fe2O3, MgO, MnO, CaO, K2O, Na2O, P2O5, and SO2 of 124 samples from this core were analyzed. The main results are as follows： （ 1 ）The average contents of major elements from BY1 follow the pattern of Si 〉 Al 〉 Fe 〉 Ca 〉 K 〉 Mg 〉 Na 〉 Ti 〉 Mn 〉 P 〉 S, and are characterized by high Si, Ti, Mn, P, S and low Na, Mg, Al, K, Ca, Fe in comparison with those of the upper continental crust in Eastern China. （2） Elements originated from the sea such as Mg, Ca, P, S are higher in marine sediments than those in continental sediments. Marine sediments are also evidenced by large numbers of foraminifera and marine ostracods. （3）For continental sediments from BY1, the average contents of Si and Mn are higher than those in the marine ones, and Al, Fe, Si are between those from the Changjiang River and the Huanghe River sediments, indicating that the detritus from these two rivers have contributed to the formation of the plain. （4） Based on sedimentary facies, microfossils, paleomagnetism, ^14C datings, and major element variations the land-sea interaction history of this part of the plain could be divided into 10 stages which are accordingly represented by the following 10 sections in the core： 1 ） 95. 22～83.95m （around 2. 58Ma）; 2） 83.95～79.66m （ 1.30 ～ 2.58Ma） ; 3 ） 79.66 ～ 75.42m ; 4） 75.42 ～ 65.40m ; 5 ） 65.4 ～ 58.0m ; 6） 58.00～39.16m（0.78～0.03Ma）; 7）39. 16～19.16m; 8） 19. 16～14.72m; 9）14.72～8.70m;10）8.7～0m. Many saltwater incursions have been recognized among which most might be caused by occasional events as floods, storms, etc., but at least four of them are major transgressions. They are around 1.95Ma（83.95 ～ 79. 66m）, 1Ma （65.4～58.0m） , 0.78Ma～30ka（39.16 ～ 14.72m） and Holocene. Transgression between 0.78Ma and 30ka is the largest one. In conclusion, it is clear from multiple lines of evidence that the evolutionary history of the inner part of the Northern Jiangsu Plain is characterized by fluvial-marine interactive events during Pleistocene and Holocene.